Webb finds a hidden atmosphere on a molten super-Earth

Their observations of the ultra-hot super-Earth TOI-561 b indicate that this exoplanet is likely enveloped in a thick layer of gases sitting above a global ocean of molten rock.

The team reported the results on December 11 in The Astrophysical Journal Letters. They explain that the new data help account for the planet’s surprisingly low density and challenge the long-standing idea that relatively small planets orbiting very close to their stars cannot hold on to atmospheres.

Extreme orbit and ultra-short period super-Earth

TOI-561 b has a radius about 1.4 times that of Earth and completes one orbit in less than 11 hours, placing it in a rare group known as ultra-short period exoplanets.

Its host star is only slightly smaller and cooler than the Sun, but TOI-561 b circles it at an extremely close distance — less than one million miles or one-fortieth the distance between Mercury and the Sun — which almost certainly locks one side permanently facing the star. On this unending dayside, temperatures are expected to soar far beyond the melting point of typical rock.

Co-author Dr. Anjali Piette, from the University of Birmingham, said: “We really need a thick volatile-rich atmosphere to explain all the observations. Strong winds would cool the dayside by transporting heat over to the nightside.

“Gases like water vapor would absorb some wavelengths of near-infrared light emitted by the surface before they make it all the way up through the atmosphere. The planet would look colder because the telescope detects less light, but it’s also possible that there are bright silicate clouds that cool the atmosphere by reflecting starlight.”

Probing the planet’s unusual low density

One possible explanation the researchers explored for the planet’s low density is that TOI-561 b might contain a relatively small iron core along with a mantle made of rock that is less dense than Earth’s interior.

Lead author Johanna Teske, staff scientist at Carnegie Science Earth and Planets Laboratory, said: “What really sets this planet apart is its anomalously low density. It is less dense than you would expect if it had an Earth-like composition.

“TOI-561 b is distinct among ultra-short period planets in that it orbits a very old, iron-poor star — twice as old as our sun — in a region of the Milky Way known as the thick disk. It must have formed in a very different chemical environment from planets in our own solar system.”

Because of this, the planet’s make-up may resemble worlds that formed when the universe itself was still relatively young.

A thick atmosphere above a global magma ocean

The team also proposed that TOI-561 b could be wrapped in a substantial atmosphere that makes the planet appear larger than its solid body alone. Although theory suggests that small planets left for billions of years in intense stellar radiation should lose any gases they once had, a few such worlds still show signs that they are not just bare rock or exposed lava.

Using Webb’s NIRSpec (Near-Infrared Spectrograph) to measure the planet’s dayside temperature based on its near-infrared brightness, researchers tested the idea that TOI-561 b has an atmosphere. The method involves tracking the drop in brightness of the combined star and planet system when the planet moves behind the star. It is similar to techniques used to search for atmospheres in the TRAPPIST-1 system and on other small rocky exoplanets.

If TOI-561 b were simply a bare rocky surface with no atmosphere to move heat to the nightside, its illuminated hemisphere should reach about 4,900 degrees Fahrenheit (2,700 degrees Celsius). Instead, the NIRSpec data indicate that the dayside temperature is closer to 3,200 degrees Fahrenheit (1,800 degrees Celsius) — still extremely hot, but significantly cooler than expected.

Testing heat transport and atmospheric scenarios

To account for this lower-than-expected temperature, the researchers evaluated several scenarios. A global magma ocean might move some heat from the dayside toward the nightside, but without an atmosphere the permanently dark side would solidify, limiting how much energy could be redistributed. A thin layer of rock vapor over the molten surface could exist, yet by itself this would not provide enough cooling to match the observations.

While the Webb measurements strongly support the presence of an atmosphere, an important puzzle remains: how can a relatively small planet that is bombarded by such intense radiation keep any atmosphere at all, especially one that appears so substantial?

Co-author Tim Lichtenberg from the University of Groningen, Netherlands, said: “We think there is an equilibrium between the magma ocean and the atmosphere. While gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior. This planet must be much, much more volatile-rich than Earth to explain the observations. It’s really like a wet lava ball.”

Webb’s long look at TOI-561 b and its broader mission

These findings come from the first results of Webb’s General Observers Program 3860, which monitored the system continuously for more than 37 hours while TOI-561 b completed nearly four full orbits of its star. The team is now studying the complete data set in detail to chart how temperature varies around the entire planet and to better pin down the makeup of its atmosphere.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).